Question: What type of mathematical function starts with the letter "r"?
Paraphrase: What sort of scientific work begins with the letter "r"?
SPARQL (Wikidata): SELECT DISTINCT ?sbj ?sbj_label WHERE { ?sbj wdt:P31 wd:Q47279819 . ?sbj rdfs:label ?sbj_label . FILTER(STRSTARTS(lcase(?sbj_label), 'r')) . FILTER (lang(?sbj_label) = 'en') } LIMIT 25 
Answer: rational function, rational function